Characterization of the sodium coupled vitamin C transporter (SVCT2) and its role in osteoarthritis

Abstract

Nutrient levels are known to influence the development of osteoarthritis (OA), presumably by modulating levels of matrix biosynthesis and degradation. These processes may be affected by ascorbic acid (AA), which is essential for post-translational modifications of normal collagen. To facilitate AA entry into the chondrocyte, a sodium dependent vitamin C transporter (SVCT2) is primarily expressed. We propose that SVCT2 may play a key role in the normal function of articular chondrocytes, and that a~terations in SVCT2 activity and expression may contribute to cartilage degeneration. This study utilized graded normal and OA human cartilage from 15 patients undergoing a total knee arthroplasty. Portions of this graded tissue were used for RNA isolation and PCR, chondrocyte isolation and in vitro experimentation, and immunofluorescent labeling of SVCT2. Analysis of the expression of SVCT2 mRNA from both healthy and OA explant tissue revealed a significant downregulation (median L\L\Ct of 3.24 cycles, 9.4-fold) of SVCT2 expression in OA tissue (p<O.OOOI). IHC staining clearly demonstrated reduced fluorescence for the SVCT2 transporter in OA tissues. The growth factor BMP-7 and inflammatory factor IL-l significantly increased AA uptake in monolayer cultures by 59% (p<0.0039) and 52% (p<0.0051) respectively. Uptake following oxidative stress (Sin-1) treatments demonstrated an increasing trend with increasing Sin-1 concentration, which was countered by AA addition with Sin-1 treatment at 600uM (p<O.OOOI) and 800uM (p<O.OOOl). Our findings, especially the explant PCR data, support the hypothesis that SVCT2 plays a role in OA, as the expression of the transporter was reduced nearly 10 fold in OA tissue. As AA is an essential nutrient to maintain cartilage integrity, this downregulation in OA implies a significant relationship. Regulatory studies demonstrate increased uptake following treatment with growth factors, inflammatory factors, and oxidative stress. This suggests a role for SVCT2 not only in anabolic processes, but in cellular responses to inflammatory and oxidative conditions as well, indicating that SVCT2 may also play a role in the cellular survival response.